The role of a plasma cell in the immune system is to produce antibodies. These antibodies are effective in binding to extracellular pathogens. Antibodies produced by plasma cells would not be effective in binding to intracellular pathogens, such as M. tuberculosis and intracellular HIV. Only one answer choice specifies an extracellular pathogen.

Plasma cells are developed from B-lymphocyte precursors in response to the presence of a specific antigen, and are part of the adaptive immune response. As such, they would be relatively ineffective at fighting a newly-evolved microbe to which the body has never been exposed before.

Cell-mediated immunity involves the response of specific cells toward an invading pathogen or organism. This is in contrast to humoral, or antibody-mediated immunity. Both of these are types of adaptive immunity, however, cell-mediated immunity involves the initial identificaiton of an unknown pathogen, while humoral immunity requires prior exposure to the antigen.

B-cells secrete antibodies and are the main actor in humoral immunity; they do not participate directly in cell-mediated immunity. Cytotoxic T-cells are activated by helper T-cells to recognize and destroy infected cells in the body. The process also involves cytokines which recruit neutrophils to digest the infected cell or microbes.

Macrophages are responsible for ingesting and degrading bacteria and viruses, and presenting their antigens on major histocompatibility complexes (MHCs) to B- and T-cells. Macrophages are thus responsible for mediating the initiation of an immune reaction.

Megakaryocytes produce platelets, and B- and T-cells are the lymphocytes to which macrophages present ingested antigens.

Endosomes function to shuttle phagocytosed material to the lysosome, where cellular digestion can take place. This means that professional phagocytes, such as macrophages, can be expected to have a larger number of endosomes than other cells that are less specialized for this process.

Megakaryocytes are responsible for producing platelets, the remnants of cells that help form clots.

B-cells and T-cells are responsible for humoral and cell-mediated immunity. Macrophages ingest bacteria and viruses and present them to B- and T-cells to initiate an immune reaction.

The immune system can be broken down into two main categories: innate and adaptive. Innate immunity includes nonspecific defense mechanisms, and the adaptive side is broken down into two primary sections, humoral and cell-mediated immunity. The key players in cell-mediated immunity are T-cells (including helper, suppressor, memory and cytotoxic T-cells). The humoral response occurs through B-cells, which are the precursors for plasma cells and memory cells. Once a B-cell is exposed to a matching antigen, it will begin to produce two types of daughter cells: plasma cells and memory cells. Plasma cells produce large amounts of antibodies in order to fight the infection at hand, where memory cells will remain in the lymph nodes for the rest of the organism's life. Memory cells are key in an organism's quick secondary response to a microbe that was previously encountered.

This question asks which of the following is NOT an antigen presenting cell, therefore, any option that is an antigen presenting cell in the immune system is an INCORRECT answer.

During an immune response, the cells involved in antigen presentation are dendritic cells, macrophages, and B-cells. T-cells, then, must be the answer choice that is NOT an antigen presenting cell, and thus is the correct answer.

In terms of MHC restriction, students should be familiar with the fact that cytotoxic T-cells are CD8+ and MHC I restricted. The alternative T subset, the helperT-ell, is CD4+ and MHC II restricted. Both cells rely on cytokines for growth, survival, and their effector functions.

The thymus is responsible for the maturation of T-cells. The thymus, a gland centered in the chest, allows T-cells to be tested against a "self-antigen." If the T-cell does not react against the self-antigen, it is allowed to leave the thymus and enter into circulation to detect foreign antigens. If the T-cell does react to the self-antigen, it is generally destroyed. Failure to destroy these T-cells can result in autoimmune attacks. Upon detection of foreign antigens, mature T-cells differentiate into a variety of other T-cell types and stimulate B-cells to produce antibodies.

CD8+ T-cells are also called cytotoxic T-cells. They are the main agents of cell-mediated immune cytotoxicity. This function is critical for the elimination of virally infected or cancerous cells. Macrophages, in contrast, are responsible for eliminating foreign pathogens and do not attack non-foreign cells.